Circuit for increasing the output of class c amplifiers



May 22, 1951 DOME CIRCUIT FOR INCREASING THE OUTPUT 0F CLASS 0AMPLIFIERS Filed Dec. 6, 1947 Fig.:. v 4

SOURCE Inven t or": RoberCBDome,

Patented May 22, 1951 UNI ED s TExs eT N1" QF CIRCUIT FOR INCREASING THEOUTPUT 0F CLASS C AMPLIFIERS Robert B. Dome, Geddes Township, OnondagaCounty, N. Y., a'ss'ignor' to General Electric My invention relates toamplifier systems, and more particularly, to amplifier systems of theclass C type in which current flows intermit tently. It is ,a primaryobject of my invention to provide improved performance characteristicsofsuchsystems.

As is well known in the art, a class C amplifier is one in which thegrid is biased appreciably beyond cutoff, so that no anode current flowsin the absence of alternating grid signal voltage, and .so that anodecurrent flows for appreciably less than one-half ,of each cycle when analternating signal Voltage is applied to the control electrode of theamplifier tube.

In power amplifiers, the operating eificiency is a primaryconsideration. Class C operation atfords the highest efficiencyobtainable, however, increased efficiency is accompanied ,b acorresponding decrease in obtainable output. An eificiency maximum of78.5% is obtainable when anode current flows during exactly one-half ofeach cycle, and 100% efiiciency is theoretically obtainable when noanode current flows. Thus, a compromise is necessitated betweenefficiency of operation and available output power.

Itis known in the art that increased efiiciency witha given output powermay be obtained if the .anodecurrentcan-be made to flow inessentiallyrectangular pulses at the time that the instantaneous anode voltage isat or near its minimum. .Various methods have been employed forproviding the desiredrectangular anode current wave- ;form, such methodscustomarily taking the form .ofa saturated positive grid voltage or ofcomplicated circuits involving auxiliary tubes.

It is a particularobject of my inventionto provide improved means forobtaining increasedefliciency of operation of a class C amplifierwhileefiiciency of a class C amplifier by squaringofi vthe tops of the anodecurrent pulses of the ampli- .fier-tube.

Aiurther object of my invention is to provide improved means forincreasing the operating effivciency of a class C push-pull electronicamplifier.

A stilliurther object of my invention'is to provide simplified means forattaining these results without consuming excessive grid driving-powerand without using auxiliary tubes.

TheI features of my invention which I believe to be novel, are set forthwith particularity in the .appendedclaims. My invention itself, however,

together 1 with further objects and advantages 5 Claims. (01. 179 -171)thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in whichFig. '1 is a schematic diagram of' an amplifier circuit which suitablyembodies my invention; Fig. is a graphical representation of theseveral'per tinent aveforms associated with the circuitiofi Fig. 1; Fig.3 is a schematic diagram'of a motile iication of that portion of thecircuit of .FigJl within the dashed rectangle A; and Fig. 4 is aschematic circuit diagram showing a further modification of myinvention. Like reference numerals indicate like elements in the severalf gures;

Referring to the circuit shown in Fig. 1, an electron discharge'device Ihavin cathode, anode, control, and screen electrodes 2', 3, 4, 5';respectively, serves as an amplifier of radio rrequency energy. A sourceof radio frequency nerg 6 connected to the control electrode 4 ofdevice] Joy means of coupling capacitor 1. The control electrode 4 isconnected to a source of negative unidirectional bias voltage, hereshown asa'bat .tery 8,.through a tuned grid circuit .9 comprising aninductance iii and 'a capacitance llfThe anode 3 of device I isconnected toi an output circuit I 2 comprising thecustomaryparallel'rso-r nant circuit l 3 coupled to a conventionalante a,oircuit 4. Positive unidirectional operating potential is applied totheanode 3 through circuit i3 froma suitable source, here shownasba'tteries l5 and 16. The cathode? of device 'I is connected directlyto ground. The customary bypass con:- densers ll, I 8, {9 are providedfor thebatteries 8, l5, [6", respectively. 'Thes'creen electrodeiofdevice .I is supplied with positive direct operating potential frombattery 15 through a tuned circuit 2 comprising an inductance '2], avariablefc apacita nce 22, an d a variable dampingresistai'iq e :23, allconnected in parallel.

In operation, the control electrode 4 of device .I

is biased beyond cutoff so that the circuit funct ions as a class .Camplifierf .With damping resistance 23 short ci rc uited, the anodecircuit .13 is tuned to ,the input'frequency in .the usual nanner, andthe antenna'cir'cuit' 44 is tunedjior optimum output. The screencircuitzptuned by means of variable capacitance v22 to a f're- .quency whichis, for purpose of example, assumed ltobethe third harnionic' of theinput irequeiicy. In certain applications, it may bedesir'able'fto tunethe screen circuit. 2} to other harmonicsi'o'i the input frequency.Damping resistance 2 ,3 "prolyides'f'a control over the magnitude of theliar.- incganic screenvoltage! As'the dampingre stance 23 is slowlyincreased from zero, a third harmonic voltage appears on the screenelectrode 5 in such a phase as to reduce the peak anode current, sincethe anode current of a screen-grid tube is a function of both controlgrid and screen grid instantaneous potentials. This reduced peak anodecurrent results in a decrease in average direct anode current in deviceI, and consequently, in decreased output power. In order to bring theaverage direct anode current of device I to its original value, thecontrol grid excitation is slightly increased. This adjustment resultsin an increase in power output while the anode dissipation remainsconstant. Hence, the operating efficiency of device I has beenincreased.

Referring to Fig. 2, there is shown a graphical representation of theinstantaneous anode current, screen voltage, and control grid voltage ofthe device I. The section to the left of the broken line B representsconventional operation of the circuit of Fig. l, with anode current ibflowing for less than one-half of the input cycle, and with a fixedscreen potential E5. The control grid bias ECO is set below the cut-offvalue Eco- It will be noted that the anode current waveform issubstantially peaked. As the third harmonic voltage es is introduced inthe screen grid circuit (shown in the section to the right of the brokenline B), the anode current is is made to flow in substantiallyrectangular pulses. As is well known in the art, optimum operatingefiiciency of a class C amplifier obtains when the anode current is madeto fiow in rectangular pulses at the time that the anode voltage is ator near its minimum. Consequently, the increase in operating efficiencydepends on the extent to which this optimum condition is approached. Thesection to the right of the dashed line B shows graphically theconditions obtained when the value of the damping resistance 23 (Fig. 1)has been adjusted to permit enough third harmonic voltage to bedeveloped at the screen electrode 5 to substantially square off the topof the anode current wave. Merely by way of illustration, one adjustmentwhich I have found to be satisfactory is to combine the controlfunctions to obtain the substantially fiat topped anode currentwaveform, andto adjust the bias voltage Ecc to such a value that anodecurrent flows during 140 degrees of the input cycle. A careful graphicalanalysis show that under these conditions the operating efiiciency is85.1%, as compared with the Well known value of 785% obtainable when theanode current fiows for one-half of the input cycle with conventionaloperation.

As a further modification, it may be desirable to introduce more thanone harmonic voltage in the screen circuit of the amplifier tube. Fig. 3is a schematic diagram of a modification of that portion of a circuit ofFig. 1 within the dashed rectangle A, the remainder of the circuit beingidentical with that of Fig. 1. For purpose of example, I have shownmeans for introducing one additional harmonic of the input frequency inthe screen circuit of device 1. Such means as shown take the form of anadditional tuned circuit 24 comprising a parallel combination of aninductance 25, a variable capacitance 26, and a variable dampingresistance 21, connected in series with the first tuned circuit 20. Inthis application the first tuned circuit 20 is tuned to the thirdharmonic and the second tuned circuit 24 is tuned to the second harmonicof the input frequency. It has been found that the introduction of thesecond harmonic as well as the third harmonic in the screen circuitresults in a still further increase in operating efficiency. It will beunderstood that harmonics other than the second and third may beemployed to advantage.

Referring to Fig. 4, there is shown in schematic form a push-pullamplifier circuit which suitably embodies my invention. I provide a pairof electron discharge amplifiers 28, 29 having respective anodes 30, 3!,cathodes 32, 33, control grids 3d, 35, and screen grids 36, 31. Thecathodes 32, 33 are connected directly to ground. Radio frequency energyis supplied to the input terminals 38, 39 of an input transformer from asuitable source, not shown. The secondary 4| of the input transformer 40comprises a parallel combination of a variable capacitance 42 and afixed inductance 43 having a center tap 44. The output terminals 45, 46of the secondary 4! are connected directly to the control grids 34, 35.A suitable source of negative direct bias voltage, here shown as abattery 41, is connected between the center tap 44 on the secondarywinding 43 and ground. The customary parallel resonant circuit 48comprising a variable capacitance 49 and a fixed inductance 50 isconnected in series with the anodes 38, 5| of amplifier tubes 28, 29.Substantially equal unidirectional operating voltages are supplied tothe respective anodes 30, 3| from a suitable source, such as batteries5!, 52 through a center tap 53 on indutance 59. I have shown aconventional antenna circuit 54 coupled to the parallel resonant anodecircuit 48, although it will be un-' derstood that the other types ofload circuits may be employed. There is provided means for introducingharmonic voltages on the respective screen grids 3B, 31, comprising atuned circuit 55 which comprises a variable capacitance 55 and a tappedinductance 51. A variable dampin resistance 58 is provided in parallelwith tuned circuit 55. Substantially equal positive direct operatingpotentials are supplied from battery 5| to the screen grids 36, 31through the center tap 59 of inductance 51. p

In operation, the bias battery 41 is of such a value that the amplifiertubes 23, 29 are biased beyond cut-off. When a radio frequency signal isapplied to the input terminals 38, 39, an ampli fied output voltageappears across the parallel resonant anode circuit 48,- the operation ofsuch a circuit being well known in the art. In order to increase theoperating efficiency, the tuned screen circuit 55 is tuned to an oddharmonic of the input frequency; this introduction of harmonic voltagein the screen circuits serves to increase the operating efficiency inthe manner heretofore described. The damping resistance 53 providesmeans for controlling the magnitude of the harmonic screen voltage. Itwill be noted that the circuit connection shown in Fig. 4 permits theintroduction in the screen circuit of odd harmonies only, since evenharmonics introduced in this manner are not in proper phase to decreasethe peak anode current. Other circuit connections may be devised incases where the introduction of even harmonics in the screen circuit isdesirable.

While I have shown and described certain present preferred embodimentsof my invention, it will be understood that numerous variations andmodifications may be made, and I contemplate, in the appended claims, tocover all such variations and modifications as fall within the truespirit and scope of my invention.

What I claim as new and .desire to secure by Letters Patent 'of theUnited States is: r

1. In an electronic amplifier of the class C type comprising an electrondischarge device having an anode, a cathode, and control and screenelectrodes, means for biasing said control electrode beyond cutofi, asource of high frequency energy connected between said control electrodeand said cathode, an output circuit connected to said anode, said outputcircuit comprising a resonant circuit tuned to the frequency of saidenergy, and means for increasing the operating efiiciency of saidamplifier comprising at least one parallel resonant circuit tuned to aharmonic of said frequency and connected between said screen electrodeand said cathode.

2. In an electronic amplifier of the class C type comprising an electrondischarge device having an anode, a cathode, and control and screenelectrodes, means for biasing said control electrode beyond cutoff, asource of radio frequency energy connected between said controlelectrode and said cathode, an output circuit connected to said anode,said output circuit comprising a resonant circuit tuned to the frequencyof said energy, and means for increasing the operating efficiency ofsaid amplifier comprising a parallel resonant circuit tuned to aharmonic of said frequency and connected between said screen electrodeand said cathode.

3. In an electronic amplifier of the class C type comprising an electrondischarge device having an anode, a cathode, and control and screenelectrodes, means for biasing said control electrode beyond cutofi", asource of radio frequency energy connected between said controlelectrode and said cathode, an output circuit connected in series withsaid anode, said output circuit comprising a resonant circuit tuned tothe frequency of said energy, and means for increasing the operatingefiiciency of said amplifier comprising a plurality of parallel resonantcircuits tuned to different harmonics of said frequency and connected inseries between said screen electrode and cathode.

4. In an electronic amplifier of the class C type comprising a pair ofelectron discharge devices connected in push-pull, each of said deviceshaving a cathode, an anode, and control and screen electrodes, means forbiasing said control electrodes beyond cutoff, a source of radiofrequency energy connected between said control electrodes and saidcathodes, an output circuit connected in series with said anodes, saidoutput circuit comprising a resonant circuit tuned to the frequency ofsaid energy, means for applying substantially equal positiveunidirectional operating potentials to said anodes, means for increasingthe operating efficiency of said amplifier comprising a parallelresonant circuit tuned to a harmonic of said frequency connected betweensaid screen electrodes, and means for applying substantially equalpositive unidirectional operating potentials to said screen electrodes.

5. In an electronic amplifier of the class C type comprising an electrondischarge device having an anode, a cathode, and control and screenelectrodes, means for biasing said control electrode beyond cutoff, asource of high frequency energy connected between said control electrodeand said cathode, an output circuit connected to said anode comprising aresonant circuit tuned to the frequency of said energy, and means forincreasing the operating efiiciency of said amplifier comprising aparallel resonant circuit tuned to an odd harmonic of said frequencyconnected between said screen electrode and said cathode.

ROBERT B. DOME.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,103,655 Whitaker Dec. 28, 19372,243,401 Sturley May 27, 1941 OTHER REFERENCES Sarbacher, Power-tubeperformance in class C amplifiers and frequency multipliers asinfluenced by harmonic voltage, pp. 607-625, Proceedings I. R. E., vol.31, No. 11, November 1943.

